What function does the slack adjuster perform in an air brake system?
- It adjusts the pressure in the brake chambers
- It ensures the brake drums are properly connected
- It adjusts the 'slack' or free play in the linkage between the push rod and the brake shoes ✓
- It modulates the air pressure throughout the brake system
Answer: C. It adjusts the 'slack' or free play in the linkage between the push rod and the brake shoes
The slack adjuster adjusts the slack, or free play, in the linkage between the push rod and the brake shoes.
Why this is the answer
The handbook describes the pushrod pushing out against the slack adjuster to apply the foundation brakes, and its adjustment pages deal with setting the running clearance between lining and drum. It lists things to check for excessive play, including slack adjuster play from a worn gear, and explains turning the adjusting bolt until the linings touch the drum, then backing off about 1/3 of a turn for correct running clearance. That is the free play in the linkage that the slack adjuster controls.
Why the other answers are wrong
- A. It adjusts the pressure in the brake chambers — Pressure in the brake chamber is set by the air system and the chamber diaphragm, not by the slack adjuster, which works on the linkage.
- B. It ensures the brake drums are properly connected — The slack adjuster is not a connector between drums; the passages place it in the linkage that the pushrod acts on to apply the foundation brakes.
- D. It modulates the air pressure throughout the brake system — Modulating air pressure through the system is not its job — the passages show it as a mechanical part adjusted to set lining-to-drum clearance.
Remember it
Think of the name: the slack adjuster's whole job is taking up the slack in the pushrod linkage, and you set it by turning the adjusting bolt until the linings touch the drum, then backing off a little.